Afterburner (engine)
From Wikipedia, the free encyclopedia
- For other uses of afterburner, see Afterburner (disambiguation).
An afterburner is an additional component added to some jet engines, primarily those on military aircraft. It was originally developed for the Miles M.52 project (during the last years of World War II) where it was called a reheat jetpipe.
Its purpose is to provide a temporary increase in thrust for situations such as take-off, or in military aircraft, combat or supersonic flight. This is achieved by injecting additional fuel into the jet pipe downstream of (i.e. after) the turbine. This fuel is ignited by the hot exhaust gasses and adds greatly to the thrust of the engine. The advantage of afterburning is significantly increased thrust; the disadvantage of afterburning is its very high fuel consumption and inefficiency but this is acceptable for the short periods in which reheat is usually used.
Jet engines are referred to as operating wet when reheat is being used, and dry when the engine is used without afterburner.
Contents |
[edit] Design
A jet engine afterburner is an extended exhaust section containing extra fuel injectors, and since the jet engine upstream (i.e., before the turbine) will use little of the oxygen it ingests, the afterburner is, at its simplest, a type of ramjet. When the afterburner is turned on, fuel is injected, which ignites readily, owing to the relatively high temperature of the incoming gases. The resulting combustion process increases the afterburner exit (nozzle entry) temperature significantly, resulting in a steep increase in engine net thrust.
In order to accommodate the resulting increase in afterburner exit volume flow, the nozzle throat area must be increased. Otherwise, the upstream turbomachinery will rematch (probably causing fan surge in a turbofan application).
[edit] Limitations
Due to their high fuel consumption, afterburners are not used for extended periods (a notable exception is the Pratt & Whitney J58 engine used in the SR-71 Blackbird). Thus, they are only used when it is important to have as much thrust as possible. This includes takeoffs from short runways (as on an aircraft carrier) and air combat situations.
[edit] Efficiency
Since the exhaust gas already has reduced oxygen due to previous combustion, and since the fuel is not burning in a highly compressed air column, the afterburner is generally inefficient compared with the main combustor. Afterburner efficiency also declines significantly if, as is usually the case, the tailpipe pressure decreases with increasing altitude.
However, as a counter-example the SR-71 had reasonable efficiency at high altitude in afterburning mode ("wet") due to its high speed (mach 3.2) and hence high pressure due to ram effect.
Afterburners do produce markedly enhanced thrust as well as (typically) a very large, impressive flame at the back of the engine. This exhaust flame may show shock-diamonds, which are caused by shock waves being formed due to slight differences between ambient pressure and the exhaust pressure. These imbalances cause oscillations in the exhaust jet diameter over distance and cause the visible banding where the pressure and temperature is highest
[edit] Influence on cycle choice
Afterburning has a significant influence upon engine cycle choice.
Lowering fan pressure ratio decreases specific thrust (both dry and when afterburning), but results in a lower temperature entering the afterburner. Since the afterburning exit temperature is effectively fixed, the temperature rise across the unit increases, raising the afterburner fuel flow. The total fuel flow tends to increase faster than the net thrust, resulting in a higher afterburning thrust-specific fuel consumption (TSFC). However, the corresponding dry power TSFC improves (i.e. lower specific thrust). The high temperature ratio across the afterburner results in a good thrust boost.
If the aircraft burns a large percentage of its fuel with the afterburner alight, it pays to select an engine cycle with a high specific thrust (i.e. high fan pressure ratio/low bypass ratio). The resulting engine is relatively fuel efficient with afterburning (i.e. Combat/Take-off), but thirsty in dry power. If, however, the afterburner is to be hardly used, a low specific thrust (low fan pressure ratio/high bypass ratio) cycle will be favored. Such an engine has a good dry TSFC, but a poor afterburning TSFC at Combat/Take-off.
Often the engine designer is faced with a compromise between these two extremes.
[edit] Usage
McDonnell F3H Demon and the Douglas F4D Skyray — were designed around the Westinghouse J-40 turbojet engine, rated at 8,000 lb. thrust without afterburner. The new Pratt & Whitney J-48 turbojet, at 8,000 lb. thrust with afterburner, would power the Grumman sweptwing fighter F9F-6, which was about to go into production. Other new Navy fighters included the highspeed Chance Vought F7V-3 Cutlass, powered by two 6,000-lb.-thrust Westinghouse J-46 engines, and the Douglas F3D Skynight, an all-weather fighter, powered by two 3,600-lb.-thrust Westinghouse J-34 turbojets
The only civilian passenger transport aircraft to use afterburners were Concorde and the Tupolev Tu-144 supersonic transport, which used them at takeoff and to minimise the time in the high drag transonic flight regime.
Except for some NASA research aircraft and the White Knight of Scaled Composites, afterburners are in the regime of military fighter jets. Modern design supercruise engines have inherently high thrust and this has lessened the need for afterburner. A turbojet engine equipped with an afterburner is called an "afterburning turbojet," whereas a turbofan engine similarly equipped is called an "augmented turbofan."
